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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 13, issue 4 | Copyright

Special issue: Low oxygen environments in marine, fresh and estuarine...

Biogeosciences, 13, 1209-1222, 2016
https://doi.org/10.5194/bg-13-1209-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 29 Feb 2016

Research article | 29 Feb 2016

Drivers of summer oxygen depletion in the central North Sea

Bastien Y. Queste1,2, Liam Fernand2, Timothy D. Jickells1, Karen J. Heywood1, and Andrew J. Hind1 Bastien Y. Queste et al.
  • 1Centre for Ocean and Atmospheric Sciences (COAS), School of Environmental Sciences, University of East Anglia, NR4 7TJ, Norwich, UK
  • 2Centre for Environment, Fisheries & Aquaculture Science (CEFAS), Pakefield Road, NR33 0HT, Lowestoft, UK

Abstract. In stratified shelf seas, oxygen depletion beneath the thermocline is a result of a greater rate of biological oxygen demand than the rate of supply of oxygenated water. Suitably equipped gliders are uniquely placed to observe both the supply through the thermocline and the consumption of oxygen in the bottom layers. A Seaglider was deployed in the shallow (≈ 100 m) stratified North Sea in a region of known low oxygen during August 2011 to investigate the processes regulating supply and consumption of dissolved oxygen below the pycnocline. The first deployment of such a device in this area, it provided extremely high-resolution observations, 316 profiles (every 16 min, vertical resolution of 1 m) of conductivity, temperature, and depth (CTD), dissolved oxygen concentrations, backscatter, and fluorescence during a 3-day deployment.

The high temporal resolution observations revealed occasional small-scale events (< 200 m or 6 h) that supply oxygenated water to the bottom layer at a rate of 2 ± 1 µmol dm−3 day−1. Benthic and pelagic oxygen sinks, quantified through glider observations and past studies, indicate more gradual background consumption rates of 2.5 ± 1 µmol dm−3 day−1. This budget revealed that the balance of oxygen supply and demand is in agreement with previous studies of the North Sea. However, the glider data show a net oxygen consumption rate of 2.8 ± 0.3 µmol dm−3 day−1, indicating a localized or short-lived (<200m or 6h) increase in oxygen consumption rates. This high rate of oxygen consumption is indicative of an unidentified oxygen sink. We propose that this elevated oxygen consumption is linked to localized depocentres and rapid remineralization of resuspended organic matter.

The glider proved to be an excellent tool for monitoring shelf sea processes despite challenges to glider flight posed by high tidal velocities, shallow bathymetry, and very strong density gradients. The direct observation of these processes allows more up to date rates to be used in the development of ecosystem models.

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In stratified shelf seas, physical and biological conditions can lead to seasonal oxygen depletion when consumption exceeds supply. An ocean glider obtained a high-resolution 3-day data set of biochemical and physical properties in the central North Sea. The data revealed very high oxygen consumption rates, far exceeding previously reported rates. A consumption–supply oxygen budget indicates a localized or short-lived resuspension event causing rapid remineralization of benthic organic matter.
In stratified shelf seas, physical and biological conditions can lead to seasonal oxygen...
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